C/C/SiC and SiC/C/SiC fibrous composites are non-brittle structural materials designed for applications in severe environments. Chemical reactivity plays a key role in their processing and their interactions with the atmosphere. During their processing by CVI, chemical reactions between the gaseous reactants/products and the fibers largely control the densification process of the fiber preform and the residual strength of the reinforcement. In a similar manner, chemical reactions control the evolution of the fibers, the interphase, the mechanical behavior and ultimately the lifetime, when the materials are exposed to oxidizing atmospheres.